Coupled and decoupled electrochemical water splitting for boosting hydrogen evolution: A review and perspective

Developing innovative water electrolysis system to realize the energy-saving hydrogen production and prevent the H2/O2 mixing is of vital importance for further boosting the sustainable development of water electrolysis for hydrogen production. Recent years have witnessed the rapid expansion of the research about the coupled and decoupled water electrolysis, in which the coupled water electrolysis system is associated with the replacement of anodic oxygen evolution reaction (OER) with kinetically favorable sacrificial agent or biomass oxidation reaction, and the decoupled water electrolysis can be described as separating hydrogen evolution reaction (HER) and OER by introducing appropriate redox mediators. To gain a deep understanding of these two interesting research fields, we here organize a comprehensive review of the recent progress of coupled and decoupled electrochemical water splitting for boosting hydrogen production. After providing a discussion about the fundamentals of decoupled and coupled water splitting and comparing with conventional water electrolysis, we have highlighted the advantages of these two systems. Subsequently, a comprehensive discussion about the application of these two systems are provided, including the structural features, reaction mechanisms, and the properties of decoupled systems. Finally, the challenges and perspectives of these two water electrolysis systems for the commercialization are manifested.